The pulse oximeter: how we measure oxygen in the blood

March 13, 2010  |  General

If you’ve ever taken your child to the doctor for any sort of breathing problem you’ve probably encountered a device called a pulse oximeter. It’s a little thing with a bright red light on it that we clip to a finger or toe. It then generates a number called the “oxygen saturation” of the blood. Using one doesn’t hurt at all. How do these things work?

The pulse oximeter really represents a breakthrough in clinical care, because it gives us a reliable, simple, and painless way to determine what the oxygen level in the blood is. We had other ways before that (and still have them), but they require sticking a needle into a blood vessel, typically the radial artery where you feel your pulse in your wrist.

The thing the machine measures, the oxygen saturation, is the proportion (percent) of the hemoglobin in the red blood cells that are carrying oxygen. Our red bloods cells are stuffed full of a protein called hemoglobin, which is a carrier for oxygen. The red cells have only one main job — pick up oxygen in the lungs, haul it around to the body where it is needed, and dump it out there. In many ways our blood stream is like a freight train with each red blood cell being one of a long string of box cars. The cars go round and round doing their job, with each red blood cell living for several months before being replaced with a fresh one.

Oxygen saturation represents the number of box cars filled with oxygen. Normally, when the train leaves the lungs, it is at least 95% filled. When it returns to pick up another load, it is still generally at least 70% filled. This provides our bodies with an important margin of error, some reserve supply of oxygen. In fact, if the train returning to the lungs is, for example, only 40% filled, this is evidence of serious problems because the reserves are being used up. The pulse oximeter, the light on your child’s finger, measures saturation of the blood leaving the lungs.

Pulse oximetry is wonderful technology. I well remember when the devices became available, because I trained in an era when we didn’t have them. Sometimes our first sign that a child might need extra oxygen was when they started to turn a dusky color, which doesn’t happen until the saturation drops to around 80%. Now we have early warning of trouble. Plus, of course, we no longer need to stick children with so many needles. That’s wonderful, too.

If you’re interested in more details about how they work, you can read about it here.

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